Wireless communication units are known. Such units comprise, among other things, two-way radios and radiotelephones, and typically include a transmitter, a receiver, and an antenna. The transmitter typically includes an information signal source and a transmit amplifier.
Wireless communication units are known to provide full duplex and half duplex operation. A communication unit that provides full duplex operation permits the communication unit to transmit (typically on a transmit frequency) and receive (typically on a receive frequency that is different than the transmit frequency) at the same time. A communication unit that provides half duplex operation requires the communication unit to transmit and receive at different times. Thus, with a half duplex communication unit, the user must stop transmitting in order to receive a communication; whereas, with a full duplex communication unit, the user can receive and transmit at the same time. In time division multiple access systems, the communication units may provide time division duplex operation that, like full duplex operation, allows the user to receive a communication without having to stop talking, but like half duplex operation, requires the actual transmission and reception of information to occur during different time periods known as time slots.
In a typical half-duplex or time division duplex communication unit, the receiver and the transmitter are intermittently coupled to the antenna via an antenna switch. The transmit amplifier and receiver portions of such a typical time division duplex communication unit 100 are depicted in block diagram form in FIG. 1. The transmit amplifier portion of the communication unit 100 includes an amplifying device 101, a direct current (DC) power supply 103 (e.g., a battery), a supply voltage controller 105, a bias controller 107, and an antenna switch 109. The receiver portion of the communication unit 100 includes a transmission line 115 having an electrical length of one-quarter wavelength of a transmit frequency of the communication unit 100, a PIN diode 119 coupled between the transmission line 115 and a signal common 121, a DC blocking capacitor 117, and a signal receiver 113.
During transmission (e.g., during a transmit time slot), an input signal 123 (e.g., modulated voice or data) is applied to the amplifying device 101, which amplifies the input signal 123 based on an applied supply voltage 125 and an applied bias voltage 127 (e.g., when the amplifying device 101 is a field effect transistor (FET)) or current (e.g., when the amplifying device 101 is a bipolar junction transistor (BJT)). The supply voltage 125 and the bias voltage 127 are provided in accordance with known techniques by applying a supply control signal 133 to the supply controller 105 and a bias control signal 131 to the bias controller 107, respectively. In addition to the supply and bias control signals, an antenna switch control signal 135 is applied to the antenna switch 109 to couple the output of the transmit amplifier's amplifying device 125 to an antenna 111 to facilitate radio transmission of the amplified signal. To prevent the amplified signal from entering (and possibly destroying) the signal receiver 113, a receive enable/disable control signal (V.sub.c) 129 is applied to the PIN diode 119 causing the PIN diode 119 to conduct current and effectively short-circuit the signal receiver end of the one-quarter wavelength transmission line 115 through the PIN diode 119. By short-circuiting the transmission line 115, the transmission line 115 effectively presents an open circuit impedance to the output of the transmit amplifier, thereby allowing substantially all of the amplified signal to be provided to the antenna 111.
During reception (e.g., during a receive time slot), the receive enable/disable control signal (V.sub.c) 129, the supply voltage 125, and the bias voltage 127 are all removed and the antenna switch control signal 135 is adjusted to cause the antenna switch 109 to open the path between the transmit amplifier and the antenna 111, thereby allowing the signal received by the antenna 111 to proceed to the signal receiver 113 via the DC blocking capacitor 117. Without the antenna switch 109 present, the output impedance of the transmit amplifier (typically 100 ohms for a gallium arsenide FET amplifying device with no supply voltage or bias voltage applied) would load the received signal, thereby reducing the magnitude of the received signal that is applied to the signal receiver 113 and, in effect, undesirably reducing the receive sensitivity of the communication unit 100.
Although, as described above, the antenna switch 109 serves the purpose of isolating the transmit and receive portions of the communication unit 100, it does so inefficiently because it introduces additional loss in the transmit path. The additional loss of the antenna switch 109 requires the amplifying device 125 to produce a higher output power than if the antenna switch 109 was not present. In order to produce the higher output power, the amplifying device 105 must (assuming little or no change in amplifier efficiency) consume additional DC power, which substantially reduces talk time. For example, for a transmit amplifier operating at an efficiency of 40% that provides 27 decibels above a milliwatt (dBm) to the antenna 109 during transmission, the transmit amplifier consumes approximately 1.25 watts of DC power if no antenna switch is present. With the antenna switch 109 present and assuming an insertion loss of 0.5 dB for the antenna switch 109, the amount of DC power consumed by the transmit amplifier increases to 1.4 watts. This increase in consumed DC power reduces the talk time by approximately 12% as compared to when no antenna switch is present.
Therefore, a need exists for a method and apparatus for increasing an output impedance of the communication unit's transmit amplifier during receive mode to eliminate the need for an antenna switch, thereby allowing the transmit amplifier to consume less DC power, while mitigating the transmit amplifier's affect on received signals.